The present invention relates to a process for removing compounds containing phenolic hydroxy groups from a gas stream containing at least one aromatic amine generated during the gas-phase hydrogenation of the corresponding nitroaromatic compounds with hydrogen, by adsorption on a basic solid. As used herein, “compounds containing phenolic hydroxy groups” are compounds that carry at least one hydroxy group on an aromatic ring. In the case of the preparation of aniline and the purification of the aniline, the impurity carrying phenolic hydroxy groups is substantially phenol itself or alternatively derivatives that, in addition to the OH function, also carry other functional groups, such as, for example, the various aminophenols.
Aromatic amines are important intermediates which must be available inexpensively and in large amounts. It is therefore necessary, for example for the hydrogenation of nitrobenzene to aniline, to build plants having very large capacities. Aniline is an important intermediate, for example for the preparation of methylenediphenyl diisocyanate (MDI), and is generally produced on an industrial scale by catalytic hydrogenation of nitrobenzene with hydrogen (See, e.g., DE-OS 2201528; DE-OS 3414714; U.S. Pat. No. 3,136,818; EP 0 696 573 B1; and EP 0 696 574 B1). In this reaction, secondary components containing phenolic hydroxy groups such as phenol itself or aminophenols, are formed in addition to the desired product aniline. These secondary components must be removed in subsequent processes by distillation before the aniline is used further. The separation of phenol and aniline represents a major challenge for the distillation technique because their boiling points are very close together. The difficulty of separating phenol and aniline is reflected in the use of long distillation columns with a large number of plates and high reflux ratios, with a correspondingly high outlay in terms of investment and energy.
More recent approaches to separating phenol and aniline describe the use of soluble alkalis for the extraction or the addition of an alkali during the distillation. For example, JP-A-49-035341 describes an alternative process in which the crude aniline is brought into contact in a fixed bed with a solid alkali material (e.g., solid sodium hydroxide) and only then passed into the distillation, or in which the distillation is carried out in the presence of the solid alkali in a concentration of from 0.1 to 3 wt. %, based on the amount of aniline to be distilled. The separation of color-critical components such as aminophenols is thereby simplified.
However, disadvantages of this process are that high molar excesses of the solid alkali are used, based on the acidic secondary components that are to be removed, and that precise dosing of the alkaline compounds is not possible. This can, in the case of overdosing, lead to corrosion problems, precipitations and highly viscous semi-solid phases in the distillation column. Underdosing may lead to incomplete removal of the critical components.
As an alternative method for removing compounds containing phenolic hydroxy groups from aniline by distillation, JP-A-08-295654 describes extraction with dilute aqueous sodium hydroxide solution (or potassium hydroxide solution). In this disclosed method, most of the phenol is transferred, in the form of sodium phenolate, into the aqueous phase, which is separated off as the upper phase by subsequent phase separation. A molar NaOH:phenol ratio in the range from 3:1 to 100:1 is required to achieve an effective reduction in the phenol content. Disadvantages of this process are the high NaOH consumption (molar excesses), the formation of alkali-phenolate-containing waste water, which results in additional disposal costs, and an additional investment for the extraction.